IV. Discussion
4.2 Patterns of microbial colonization and carbon translocation in the rhizosphere of different
microcosms based on other inocula than rice paddy soil was even higher compared to this based on the rice paddy soil-system.
4.2 Patterns of microbial colonization and carbon translocation in the
propionate originating from the conversion of root released carbon compounds, can be further degraded to acetate, CO2, and H2 (Krylova et al., 1997) and consequently provide precursors for the formation of methane (Conrad, 2002; Schink and Stams, 2013). Afterwards, methane may be transported to the atmosphere via the aerenchyma tissue of the rice plant (Minoda and Kimura, 1994; Nouchi et al., 1990) or oxidized by methane-oxidizing bacteria colonizing the rhizosphere (Frenzel, 2000). However, in this study only short chain fatty acids of acetate, propionate, formate, and pyruvate could be detected. Since no enrichment of 13C could be detected in either formate or pyruvate in any of the microcosms, it was assumed that rhizodeposits degraded to formate and pyruvate were rapidly converted further to CO2 and acetate, or that root derived carbon did not contribute to their formation at all. As has been mentioned before (see 4.1), the translocation of recently assimilated carbon to porewater substances and methane was assumed to have exclusively originated from the release of rhizodeposits by the rice plant.
Microcosms based on rice paddy soil-system showed a lower bacterial colonization of the rhizosphere compared to the other microcosms. Nevertheless, the rhizosphere was colonized by bacteria and their absolute abundance was slightly higher in the rhizospheric soil than in the soil before planting. Although the bacterial colonization in the rice paddy soil-system was low, these microcosms showed by far the highest translocation of root derived carbon to porewater propionate, acetate, and CO2 (Figure 3.7). In contrast to the other soil-systems, porewater CO2 in the rice paddy soil-system showed a stable 13C enrichment at its maximum, indicating some availability of rhizodeposits in the preceding carbon pools. The conversion rate of recently root derived carbon to acetate was higher than in the other soil-systems, and the conversion rate to propionate was also high. Based on carbon translocation to the rhizosphere, the rice paddy soil-system is thought to posses a bacterial community composition which was able to degrade root derived carbon more effectively than that of the other microcosms and/or had a higher dependency on rhizodeposits since initial soil organic carbon in this soil-system was the lowest (Table 3.2).
Whereas colonization by methanogenic archaea in the rice paddy soil-system seemed to take place mostly on the root surface, colonization by methane-oxidizing bacteria primarily took place in the rhizospheric soil. The absolute abundance of methanogens in the rhizospheric soil was lower compared to the other microcosms, while that on the root surface was at the same level as in the other soil-systems. According to the low number of methanogens in the soil before planting, the methanogenic community of the rice paddy soil-system either seems to be quite capable of colonizing the root surface, or had a higher need of spatial proximity to root derived carbon, due to
the low amount of organic carbon in the initial soil-system. However, in the rice paddy soil-system, both the absolute abundance of methanogenic archaea in the entire rhizosphere and the total emission rate of methane (Figure 3.6) were low. In contrast to this, the enrichment of 13C of methane (Figure 3.7), as well as the contribution of carbon recently assimilated by the plant to the total methane emission (Figure 3.9) was by far the highest in the rice paddy soil-system.
Therefore, the methanogenic community composition of this soil-system is thought to use intermediates originating from the degradation of root derived carbon more effectively and/or to have a higher dependency for rhizodeposits because of the low amount of initial soil organic carbon in this system. Because of the relation of the rice plant and the inoculated rice paddy soil, it might be possible that the bacterial as well as the methanogenic community of the rice paddy soil-system were better adapted for the degradation of rice root derived carbon to methane. However, the emission of methane originating from root derived carbon was also high, but just as high as in the case of the digested sludge soil system.
For microcosms based on the mixed inoculum soil-system, bacterial colonization of the rhizospheric soil was lower compared to the soil before planting and was in general as low as in the rice paddy soil-system. Therefore, the bacterial community of the mixed inoculum soil-system had either a low capability or a low necessity for colonization of the rhizospheric soil. In contrast to this, the bacterial colonization of the root surface was high. However, as opposed to the rice paddy soil-system, microbial translocation of root derived carbon to propionate, acetate and CO2 was much lower in microcosms based on the mixed soil-system. Conversion rates of rhizodeposits to propionate were also lower than in the other microcosms. Considering this, the initial soil organic carbon seems to be a more important source for bacterial degradation of organic matter in the mixed inoculum soil-system than rhizodeposits. Therefore, the rhizosphere of the mixed inoculum soil-systems is thought to posses a bacterial community composition which is able to degrade the initial soil organic carbon more effectively and/or to have almost no need for rhizodeposits due to the high amount of soil organic carbon in this system. However, the mixed inoculum also possessed a contingent of inoculated rice paddy soil. Therefore, if the above assumption is correct and bacterial groups in the rice paddy soil were better adapted to the rice root, these groups may be absent or of low abundance in the rhizosphere of the mixed soil-system, or they did not contribute to the conversion of root derived carbon at all due to the higher availability of initial soil organic carbon.
The colonization by methanogenic archaea in the mixed soil-system was similar for rhizospheric soil, root surface, and soil before planting, which indicates no considerable preference of colonizing the rhizosphere. Considering this as well as the fact that the bacteria seemed to feed more on initial soil organic carbon rather than on rhizodeposits, intermediates of degradation usable for methanogenesis might not be more available in the rhizosphere compared to the rest of the soil.
Nevertheless, absolute abundance of methanogens on the root surface was at the same level as in the rice paddy soil-system and even slightly higher in the rhizospheric soil, which would in turn be compatible with the higher total CH4 emission from the mixed soil-system. The enrichment of 13C within methane as well as the contribution of recently plant-assimilated carbon to the total methane emission were much lower than in the rice paddy soil-system, but on the same level as the digested sludge soil-system. The emission of methane originating from root derived carbon was also lower than in the other microcosms. Therefore, the methanogenic community of the mixed soil-system is thought to use intermediates originating from the degradation of initial soil organic carbon more effectively and/or to have a lower dependency on rhizodeposits due to the high amount of initial soil organic carbon in this system. Furthermore, methane-oxidizing bacteria showed a preference for the colonization of the rhizospheric soil rather than for the other soil compartments. Since the mixed inoculum was a combination of rice paddy soil and digested sludge, colonization of the rhizosphere could be assumed to be an average between the rice paddy soil-system and the digested sludge soil-system. However, it has to be mentioned that the determination of microbial abundance in the soil before planting took place at a different point in time then for rhizospheric soil and the root surface. Hence, the abundance of microorganisms capable of colonizing the rhizosphere might have developed differently between these two points in time. Therefore, some microorganisms might not have been available for colonizing the rhizosphere. Furthermore, it could be assumed that the same microorganisms which were colonizing the rhizospheric soil in the rice paddy soil-system were less dependent on doing so in the mixed inoculum because of the higher amount of initial soil organic carbon. The abundance of the microorganisms colonizing the rhizospheric soil of the digested sludge contingent was, however, decreased due to a dilution caused by the mixture of these two inocula.
The absolute abundance of bacteria in the rhizospheric soil of microcosms based on the digested sludge soil-system was higher compared to the other soil-systems. Although the absolute abundance of bacteria was the highest of all microcosms in the entire rhizosphere, the number of bacteria was high even before planting and therefore no differences with regard to colonization could be observed for colonization between rhizospheric soil and soil before planting. Despite the
high number of bacteria in the entire rhizosphere, microbial degradation of rhizodeposits to propionate and acetate was in between that of the rice paddy soil-system and that of the mixed inoculum soil-system. The contribution of root derived carbon to the formation of acetate and propionate was higher in the digested sludge than in the mixed inocula microcosms, but lower than in those based on the rice paddy soil-system. Therefore, the rhizosphere of the digested sludge soil-system was thought to posses a bacterial community composition which is able to degrade root derived carbon but not as effectively as in the rice paddy soil-system, despite also having the highest amount of initial soil organic carbon.
The colonization of the rhizospheric soil by methanogenic archaea was far higher in the digested sludge soil-system compared to the other microcosms, while the absolute methanogenic abundance on the root surface was the same as in the other soil-systems. Furthermore, methane-oxidizing bacteria also preferred the rhizospheric soil of the digested sludge microcosms for colonization, and their abundance on the root surface was far higher than in the other microcosms and in the soil before planting. In the microcosms based on the digested sludge system, methanogenic archaea clearly preferred colonizing the rhizospheric soil, since their presence on the root surface was even lower than in the soil before planting. In accordance to the highest absolute abundance of methanogens in the entire rhizosphere, the total methane emission from these microcosms was also far higher.
Despite the numerous occurrence of methanogens in the rhizosphere, 13C enrichment within methane, as well as the contribution of recently derived plant carbon to the total emission of methane, was as low as in the mixed inoculum soil-system. Nevertheless, emission of methane originating from root derived carbon was as high as in the rice paddy soil-system. Therefore, the rhizosphere of the digested sludge soil-system was thought to posses a bacterial and methanogenic community composition which is able to degrade root derived carbon to methane, but not as effectively as in the rice paddy soil-system. Furthermore, rhizodeposits play a role in the formation of methane in microcosms based on the digested sludge soil-system, despite the fact that the digested sludge soil-system had the highest amount of initial soil organic carbon.
Considering all of this, the potential of the rhizospheric bacterial community to degrade root derived carbon to precursors for formation of methane could not just be explained by the absolute abundance of rhizospheric bacteria, nor by the amount of initial soil organic carbon of the different soil-systems. The same holds true for the absolute abundance of methanogenic archaea and the formation of methane from root derived carbon. Nevertheless, microcosms with a high emission
rate of total methane also possessed a high number of methanogenic archaea in the rhizosphere, indicating a link between colonization of the rhizosphere by methanogenic archaea and the formation of methane. These outcomes were contrary to the results of (Pump et al., 2015), which showed the abundance of methanogenic archaea to increase linearly with the emission of methane originating from root derived carbon, but rates of total methane could not be systematically related to the number of methanogens. It has to be mentioned that a correlation between the abundance of methanogenic archaea and the emission of methane originating from root derived carbon of those previous study could only be determined until the vegetative growth stage. Since the rice plants of our study were considered to be at the reproductive stage, the turning point of this correlation may already have been exceeded. Furthermore, the relation between the total emission of methane and the number of methanogenic archaea was investigated with regard to the methanogenic abundance of the soil plus the roots, whereas that of the rhizosphere was considered in our study. However, since the translocation of recently assimilated plant carbon to the rhizosphere is affected by the microbial soil community (Pump and Conrad, 2014), it is thought that the degradation of root derived to methane depends more on the bacterial and methanogenic community composition and not merely on the absolute abundance in the rhizosphere or on the amount of initial soil organic carbon.
Furthermore, in almost none of the different rhizospheric compartments of all the microcosms, the absolute abundance of bacteria, methanogenic archaea, or methane-oxidizing bacteria was higher with respect to the other microcosms, if not already also in the soil before planting. Therefore, microbial colonization of the rhizosphere by bacteria and methanogenic archaea is thought to depend on the absolute abundance of the initial soil microbial communities of the different soil-systems.